Henry David Thoreau (2014). “Citizen Thoreau: Walden, Civil Disobedience, Life Without Principle, Slavery in Massachusetts, A Plea for Captain John Brown”, p.20, Graphic Arts Books
Theodore Roosevelt, Eugene Thwing (1919). “Newer Roosevelt messages: speeches, letters and magazine articles dealing with the war, before and after, and other vital topics”
Jean Baptiste Say, Charles Robert Prinsep (1857). “A Treatise on Political Economy; Or, The Production, Distribution, and Consumption of Wealth”, p.439